1. Field of the Invention
The present invention relates to an automotive emission control system suitable for use in internal combustion engines, and specifically to an electronic centralized engine control system which performs various engine controls, such as an exhaust-gas-recirculation control, a fuel injection control, a precise detection of flow rate of intake air drawn into an intake manifold or the like.
2. Description of the Prior Art
As is generally known, in order to minimize or eliminate atmospheric pollution from automotive vehicles, there have been proposed and developed various automotive tune-up and exhaust-emission control techniques. For example, an exhaust-gas-recirculation control system, often abbreviated simply as an "EGR system", is used to reduce nitrogen oxide (NOx) emissions from exhaust gases of the internal combustion engine. On late-model diesel-engine cars, the EGR system is almost all employed to decrease formation of NOx. In typical EGR systems, a target EGR rate or a target EGR amount is determined depending on engine operating conditions, namely engine speed and load on the engine. The engine load can be generally estimated by a fuel-injection amount, an opening of an accelerator (an accelerator-pedal position) or the like. One such prior art EGR system has been disclosed in Japanese Patent Provisional Publication No. 58-35255. In an EGR system incorporated in diesel engines, it is desired to properly and precisely control the amount of exhaust-gas recirculation (EGR) or the rate of EGR, in a transition state for example a transition from normal straight-ahead driving of the vehicle to heavy acceleration, or in case of changes in air density resulting from changes in environment from low-land driving to high-land driving even under constant engine speed and engine load. Also, in a diesel engine with a turbocharger, there is the additional problem that the acceleration versus super-charged induction pressure (often called boost pressure) characteristics is affected by degradation of lubricating oil for lubrication of the same turbine-wheel shaft as the compressor-pump rotor. As is well known, in the event that the amount of EGR becomes excessively great, black smoke and particulates tend to increase. On the other hand, in the event that the amount of EGR is excessively less, the combustion temperature cannot be lowered satisfactorily owing to less inert exhaust gases recirculated, and thus the amount of NOx emissions cannot be sufficiently reduced. Particularly when quickly accelerating, the fuel-injection amount tends to be rapidly increased, and thus an excess-air factor tends to be greatly lowered, thereby resulting in increase in emissions of smoke and particulates. In order to avoid the undesired lowering of excess-air factor, resulting from the rapid increase in fuel injection amount when quickly accelerating, the exhaust-gas recirculation would be intendedly cut. The conventional EGR system could not timely perform the EGR-cut operation in the previously-noted transition state. The improper EGR-cut timings produce the increased amount of nitrogen oxide emissions (in case of lack of the EGR) or the increased amount of smoke and particulates (in case of excessively increased amount of the EGR). Specifically, in the case of an engine with a turbocharger, there is a greatly increased tendency for the previously-described improper EGR-cut timings to occur due to fluctuations in the acceleration versus super-charged induction pressure characteristics resulting from degraded lubricating oil. To ensure the EGR control or to reduce NOx emissions in the transition state such as during acceleration, the prior art EGR control system is equipped with an induction-air throttle valve and/or an exhaust throttle valve to properly adjust the differential pressure between the intake pressure and the exhaust pressure and consequently to adjust the EGR rate toward a target EGR rate. For example, Japanese Patent Provisional Publication No. 60-219444 has taught the provision of an EGR control which is in dependent on acceleration (or a rate of change in engine load). According to the EGR control disclosed in the Japanese Patent Provisional Publication No. 60-219444, an exhaust throttle valve is shifted to its fully-open position when the rate of change in engine load is held greater than a predetermined threshold for a preset period of time. However, in case of an engine with a turbocharger, the optimal EGR rate would vary depending on degradation of lubricating oil as well as the engine load. Japanese Patent Provisional Publication No. 60-222551 has taught the provision of an exhaust throttle valve control based on a back pressure measured upstream of the exhaust throttle valve. According to the Japanese Patent Provisional Publication No. 60-222551, the opening of the exhaust throttle valve is adjusted depending on the deviation between the back pressure measured and a target back pressure which is predetermined by both engine load and engine speed, such that the measured back pressure is adjusted towards the target back pressure. As may be appreciated, it is troublesome to precisely preset control characteristics of openings of the intake throttle and/or the exhaust throttle, because the control characteristics are affected by characteristics of an EGR control valve, different operating requirements of the engine and the like. In order to avoid excessive lowering of excess-air factor during acceleration of the vehicle, Japanese Patent Provisional Publication No. 58-138236 has taught the stepwise increasing adjustment of a fuel injection amount from the time when the vehicle begins to accelerate. Actually the fuel injection amount and/or the fuel-injection timings must be varied depending on the presence or absence of exhaust-gas recirculation (EGR) or on the EGR rate. In the system disclosed in the Japanese Patent Provisional Publication No. 58-138236, assuming that the fuel-injection amount and timings are adjusted to meet in the presence of the EGR during acceleration, the fuel-injection timing tends to delay in absence of the EGR, thus making a sacrifice of an acceleration performance. In contrast to the above, assuming that the fuel-injection amount and timings are adjusted to meet in the absence of the EGR during acceleration, the excess-air factor tends to be excessively lowered in presence of the EGR, thus increasing exhaust emissions such as black smoke and particulates. As appreciated, it is important to precisely detect or measure a flow rate of intake air or induction air which is drawn into an intake manifold. As is generally known, on late-model cars, a precise measurement of intake air is required to determine a fuel-injection amount in case of an electronically-controlled fuel-injection system for gasoline engines, and to determine a maximum fuel-injection amount in case of an electronically-controlled fuel-injection system for diesel engines. In recent years, a hot-wire type air-flow meter is widely used to detect the flow rate of intake air flowing through the air-intake pipe disposed just downstream of an air cleaner. The hot-wire type air-flow meter is inexpensive and has a relatively wider dynamic range for flow-rate measurement. Owing to a so-called valve overlap during which the open periods of both intake and exhaust valves are overlapped, the intake valve opens from before completion of the exhaust stroke, that is, prior to the top dead center (T. D. C.) position and the exhaust valve remains open after the T. D. C. position. During the valve overlap, there is a possibility of back-flow or reverse-flow of some of intake air drawn into the intake-valve port. Particularly in case of a low flow rate of intake air or a high engine load, there is a tendency for pulsation flow of intake air or pulsation of the manifold pressure to occur. The previously-noted conventional hot-wire type air-flow meter can measure the flow rate of intake air but not detect directions of the intake-air flow. The flow rate of air flowing from the intake-valve port back to the intake manifold would be measured erroneously as a positive flow rate. Thus, in case of occurrence of pulsation flow resulting from a low flow rate of intake air, there is a tendency that the measured value of intake air may be increased as compared with the actual flow rate. The erroneously-measured flow rate of intake air may exert a bad influence on the fuel-injection control, and whereby the engine performance or the driveability of the vehicle may be degraded. In diesel engines, such a measurement error results in deterioration in an exhaust-emission control performance, because the target EGR rate is usually determined depending on the differential pressure between the exhaust pressure and the intake pressure (or the manifold pressure). For example, in the case that the measured value of the air-flow meter exceeds an actual intake-air flow rate, the EGR rate is set at a greater value than an optimal EGR rate, thus increasing exhaust emissions for example particulates. Conversely, in the case that the measured value is less than the actual intake-air flow rate, the EGR rate is set at a less value than the optimal EGR rate, thus increasing the amount of NOx emissions. In these cases, the emission control performance tends to be lowered. Also to avoid an erroneous measurement of the intake-air flow rate, occurring due to pulsation of the manifold pressure during high engine load, Japanese Patent Provisional Publication No. 57-56632 has taught the use of an estimate of intake-air flow in place of a measured value of the hot-wire type air-flow meter during the high engine load, i.e., when the throttle opening exceeds a predetermined threshold value. The estimate of intake-air flow is preset on the basis of both a throttle opening and an engine revolution speed. It is desirable that the deviation (the error) between the actual flow rate of intake air and the estimate of intake-air flow is less as much as possible. However, when there are changes in air density due to a change in driving condition from low-land driving to high-land driving, the previously-noted deviation tends to increase. The increased deviation may exert a bad influence on an accuracy of the fuel-injection control or the EGR control.